1. Field of the Invention
The present invention relates to an image fixing unit for use in a wet-type electrophotographic copying machine.
2. Discussion of Background
In a wet-type electrophotographic copying machine, there is conventionally known an image fixing method comprising the steps of developing a latent electrostatic image formed on a photoconductive member to a visible toner image with a liquid-type developer, transferring the visible toner image to a transfer sheet, and fixing the transferred image to the transfer sheet by bringing heat application means into direct contact with the toner-image-bearing surface of the transfer sheet.
As shown in FIG. 1, for example, a heat-application roller 1 with an inner heater 3 serving as a heat-application means incorporated therein is disposed along a paper transportation path through which an unfixed toner-image-bearing transfer sheet P advances in the direction of the arrow, and a pressure-application roller 2 is in pressure contact with the heat-application roller 1. More specifically, a cam surface of a pressure-application cam 5 is in pressure contact with a shaft portion of the pressure-application roller 2 by the urging force of a spring 6, so that a predetermined pressure is applied to the pressure-application roller 2, thus the pressure-application roller 2 is brought into pressure contact with the heat-application roller 1. Around the heat-application roller 1, there are provided a thermistor 7 and a temperature-control fuse 8 to control the temperature of the heat-application roller 1. A transfer sheet separating pawl 10 and a silicone-oil-application felt 11 are situated in contact with the surface of the heat-application roller 1, so that the transfer sheet is prevented from being wound around the heat-application roller 1.
However, the above-mentioned conventional image fixing unit for use in an electrophotographic copying machine has the shortcoming in that the image quality of fixed image varies depending on the kind or type of a transfer sheet P employed. In particular, when transfer sheets which scarcely absorb a carrier liquid contained in a liquid developer, that is, transfer sheets having a low carrier liquid absorbing capacity, such as a transparent film for overhead projectors (OHP), an intermediate original sheet and art paper, are employed, an unfixed toner image on the transfer sheet is caused to corrupt or flow in the course of image fixing because of the presence of the excess carrier liquid remaining in the developed toner image and accordingly the fixed image becomes illegible.
In order to solve the above problem, an image fixing test was carried out to analyze the relationship between a carrier liquid retention ratio in a developer deposited imagewise on a variety of transfer sheets and the occurrence of image flow in the obtained toner images.
The above-mentioned carrier liquid retention ratio of the developer was obtained according to the following formula: ##EQU1##
To obtain the above-mentioned carrier liquid retention ratio of the employed developer, a liquid-type developer was prepared by dispersing and diluting a thermal cross-linking toner in a carrier liquid such as "Isopar H" made by Exxon Co., with a dilution ratio of 50 g/l. Latent electrostatic images were developed to toner images on a commercially available transparent film for OHP (Trademark "TYPE PPC-DX" (A4 size) made by Ricoh Company Ltd.) by use of the above-mentioned developer and a commercially available electrophotographic copying machine. The thus developed OHP film was taken out from the electrophotographic copying machine before reaching an image fixing unit of the copying machine. The weight of the OHP film was measured. After this measurement, the OHP film was then dried through the image fixing unit to evaporate the carrier liquid, and the weight thereof was measured, so that the carrier liquid retention ration (%) was obtained in accordance with the above-mentioned formula. In the above-mentioned image fixing test, the fixing temperature was adjusted to 140.degree. .+-.10.degree. C. and the paper feeding linear speed was set at 350 mm/sec.
The results are set forth in Table 1.
TABLE 1 ______________________________________ Carrier Image Flow Liquid Intermediate Art Plain Retention OHP Original Paper Paper Ratio (%) Film (*) Sheet (**) (***) (T-6200) ______________________________________ 77 1 1 2 5 72 2 2 3 5 68 2 3 3 5 65 2.5 3 3.5 5 60 3 3 4 5 55 4 4 5 5 45 4.5 5 5 5 9 5 5 5 5 ______________________________________ (*) A commercially available transparent film for the OHP "TYPE PPCDX" made by Ricoh Company Ltd. (**) A commercially available intermediate original sheet "TYPE TA" made by Ricoh Company Ltd. (***) A commercially available duplex coated paper.
In Table 1, the image flow was evaluated by visual inspection and the degree of the image flow is expressed by the following ranking scale:
1: Image flow is observed in all the obtained images so that the characters are illegible. PA1 2: Image flow is slightly observed in the obtained images and the image density of the solid image areas is low. PA1 3: Image flow occurs less, but the characters in the solid image areas spread. PA1 4: Image flow is hardly observed, but the characters in the solid image areas slightly spread. PA1 5: Image flow is not observed at all and the characters in the solid image areas are clear.
The results in Table 1 indicate that when the carrier liquid retention ratio in the developer deposited on the transfer sheet is 65% or more, image flow takes place to some extent in the fixed images on the OHP film, the intermediate original sheet and the art paper.
However, a satisfactory image fixing unit capable of attaining effective image fixing for use in a wet-type electrophotographic copying machine, by controlling the above discussed carrier liquid retention ratio, has not been proposed.
In addition, the conventional image fixing units have the shortcoming that the durability of the heat-application roller 1 is low. More specifically, the surface of the heat-application roller 1 is easily scratched or damaged by the transfer sheet separating pawl 10, in particular when the heat-application roller 1 is coated with silicone rubber, because the transfer sheet separating pawl 10 is always in contact with the surface of the heat-application roller 1.
Another shortcoming of the conventional image fixing units is that the effect of the silicone-oil-application felt 11 is readily deteriorated with time. As shown in FIG. 1, the silicone-oil-application felt 11 is situated in contact with the surface of the heat-application roller 1 and serves to uniformly apply silicone oil to the surface of the heat-application roller 1 in order to facilitate the separation of the transfer sheet from the heat-application roller after image fixing. However, some foreign materials such as offset toner particles and paper dust are gradually deposited and accumulate on a tip 11a of the silicone-oil-application felt 11 as shown by a shaded portion 11b each time the transfer sheet is caused to pass through the image fixing unit. Eventually the silicone-oil-application felt 11 does not work effectively. Therefore periodical replacement of the silicone-oil-application felt 11 is unavoidable.
Furthermore, in the case where a silicone oil is supplied to the heat-application roller 1 by a silicone-oil-application roller (not shown) and the above-mentioned foreign materials are removed from the heat-application roller 1 by a scraper blade (not shown) which is disposed in contact with the surface of the heat-application roller 1, if the heat-application roller 1 is rotated in the same direction as the rotating direction of the silicone-oil-application roller, that is, when the movements of the surfaces of the two rollers are opposite at their contact point, the foreign materials accumulated on the tip of the scraper blade are transferred back to the heat-application roller 1, so that not only the surface of the heat-application roller 1, but also the transfer sheets are smeared by the retransferred foreign materials.